The present invention relates to a thermal printer which can be selectively set into a high resolution printing mode or a high speed printing mode.
Hitherto, in wire dot printers, the pitch between dots has been set to 1/72 inch and this pitch has generally been used in the world. Therefore, in most printers, it is required to set the lowest resolution of a graphic pattern which is printed into 1/72 inch.
On the other hand, in the printer such as a thermal printer which requires a high resolution, to realize the printing at the pitch of 1/72 inch between dots, it is necessary to set the dot pitch of the thermal head into a 1/72.times.n (n is an integer of 2 or more) inch, for example, 1/144, 1/216, or 1/288 inch. In this case, the number of heating elements which are used to constitute one dot increases with a decrease in n. When considering the practical use of the thermal printer, the high resolution characters printed by the thermal head having the heating elements which are arranged at the pitch of 1/144 inch between dots are slightly roughly observed. On the other hand, when the thermal head having the heating elements arranged at the pitch of 1/216 inch between dots is used, the high resolution characters having a high near-letter-quality can be printed with a high degree of fidelity. However, since it is necessary to arrange a number of heating elements in the limited space, it is difficult to manufacture such a kind of thermal head and its circuit arrangement also becomes complicated. Therefore, it is considered that it is advantageous to arrange the heating elements at the pitch of 1/180 inch between dots. Namely, it is desirable to perform the high density printing or high resolution printing at the dot pitch of 1/180 inch and execute the ordinary high speed printing at the dot pitch of 1/72 inch. However, hitherto, it has been considered that in the case where the dot pitch of 1/180 inch is used for the high density printing, only the dot pitch of 1/190 or 1/60 inch can be used for the high speed printing. Therefore, it has been considered that two sets of thermal heads are prepared and used by switching as necessary in order to effect the printing at the dot pitch of 1/180 or 1/72 inch. The use of two sets of thermal heads, however, is disadvantageous in terms of costs and causes the structure of printer to become complicated.